This invention relates to thin film conductors and, in particular, to improved thin film stripes having high resistance to electromigration.
Thin, narrow stripes formed of a conductive material such as aluminum have been used for some time in solid state devices to interconnect various types of system related components. Similar conductive stripes are also now being used in magnetic bubble memories. As the state of the art in these integrated circuit applications approaches Very Large Scale Integration (VLSI), the physical size of the interconnects become correspondingly small and the ability of the interconnect to resist electromigration damage (EMD) becomes increasingly important. As the size of the interconnect approaches micron and submicron dimensions, the current density carried by the stripe increases to a point where a mass transport of atoms takes place resulting in the creation of voids in one area of the stripe and a thickening of the conductor in a second area downstream from the first in the direction of current flow. Generally at current densities below 10.sup.4 amp/cm.sup.2 electromigration has little effect on the life of a thin film conductor. However, at current densities of about 10.sup.5 amp/cm.sup.2 and above, EMD takes place and reduces the life expectancy of the circuit. Failure can be produced by an open circuit being created in the void region or a short circuit being generated in the thickened area as the expanded material comes in contact with another conductor. It should be further noted that many thin film conductors of this type are provided with a passivation layer such as glass which becomes fractured by the material build-up. The fracture can expose the electronic component to a hostile atmosphere thus further inducing early failure of the current carrying device.
It has been noted by many investigators that a hydrogen containing ambient considerably improves the electromigration behavior of a thin film conductor. Evidence has been developed that a substantial reduction in solid state transport can be achieved through interaction of active gases such as hydrogen with thin film conductors or bimetallic diffusion couples. The rate of intermetallic compound formation is reduced dramatically in diffusion couple devices when the device is placed in a hydrogen environment rather than air. See D. Y. Shih and P. J. Ficalora, Proc. IEEE International Reliability Phys. Sympos. pgs. 87-90 (1979). It has also been noted that the mean time to failure (MTF) of a thin film conductor is increased considerably when an aluminum stripe is vacuum deposited in an atmosphere containing hydrogen. See for example, D. E. Meyer, Journal of Vacuum Science Technol., Vol. 17 pgs. 322-326 (1980).
The technological importance of a hydrogen ambient is now apparent. It can also be shown that a substantial improvement in electromigration behavior is realized by use of this technique when compared to results obtained by changing the composition of the conductor. U.S. Pat. Nos. 4,352,239; 4,017,890; 4,166,279; 4,268,584 and 3,725,309 all represent various compositional arrangements that are utilized for the purpose of suppressing electromigration in conductive stripes. However, to date little is known about the mechanism by which either compositional or ambient changes actually affect electromigration in a micron or submicron conductor.